EUV-mirror, optical system with EUV-mirror and associated operating method

ABSTRACT

An EUV mirror ( 1000 ) has a mirror element which forms a mirror surface of the mirror. The mirror element has a substrate ( 1020 ) and a multilayer arrangement ( 1030 ) applied on the substrate and having a reflective effect with respect to radiation from the extreme ultraviolet range (EUV). The multilayer arrangement has a multiplicity of layer pairs having alternate layers composed of a high refractive index layer material and a low refractive index layer material, has an active layer ( 1040 ) arranged between a radiation entrance surface and the substrate and consisting of a piezoelectrically active layer material, the layer thickness (z) of which active layer can be altered by the action of an electric field, and has an electrode arrangement to generate the electric field acting on the active layer.

The present application is a Continuation of U.S. patent applicationSer. No. 14/034,069 filed Sep. 23, 2013, which is a Continuation ofInternational Application No. PCT/EP2012/055013, filed on Mar. 21, 2012,which claims priority of German Patent Application No. 10 2011 005940.7, filed on Mar. 23, 2011, U.S. Provisional Application No.61/466,631, filed on Mar. 23, 2011, German Patent Application No. 102011 077 234.0, filed on Jun. 8, 2011, and U.S. Provisional ApplicationNo. 61/494,718, filed on Jun. 8, 2011. The disclosures of these sixapplications are hereby incorporated herein by reference in theirrespective entireties.

FIELD OF AND BACKGROUND OF THE INVENTION

The invention relates to an EUV mirror, to an optical system comprisingan EUV mirror, and to a method for operating an optical system. Onepreferred field of application is EUV microlithography. Other fields ofapplication are in EUV microscopy and EUV mask metrology.

Nowadays predominantly microlithographic projection exposure methods areused for producing semiconductor components and other finely structuredcomponents. In this case, use is made of masks (reticles) or otherpatterning devices which carry or form the pattern of a structure to beimaged, e.g. a line pattern of a layer of a semiconductor component. Thepattern is positioned in a projection exposure apparatus between anillumination system and a projection lens in the region of the objectsurface of the projection lens and illuminated with an illuminationradiation provided by the illumination system. The radiation altered bythe pattern passes as projection radiation through the projection lens,which images the pattern onto the substrate to be exposed, which iscoated with the radiation-sensitive layer.

The pattern is illuminated with the aid of an illumination system,which, from the radiation from a primary radiation source, forms anillumination radiation which is directed onto the pattern and which ischaracterized by specific illumination parameters and impinges on thepattern within an illumination field of defined form and size. Withinthe illumination field, a predetermined local intensity distributionshould be present, which is normally intended to be as uniform aspossible.

In general, depending on the type of structures to be imaged, differentillumination modes (so-called illumination settings) are used, which canbe characterized by different local intensity distributions of theillumination radiation in a pupil surface of the illumination system. Itis thereby possible to predetermine in the illumination field a specificillumination angle distribution or a specific distribution of theimpinging intensity in the angle space.

In order to be able to produce ever finer structures, various approachesare pursued. By way of example, the resolution capability of aprojection lens can be increased by enlarging the image-side numericalaperture (NA) of the projection lens. Another approach consists inemploying shorter wavelengths of the electromagnetic radiation.

If it is attempted to improve the resolution by increasing the numericalaperture, then problems can arise by virtue of the fact that as thenumerical aperture increases, the depth of focus (DOF) that can beachieved decreases. This is disadvantageous because a depth of focus ofthe order of magnitude of at least 0.1 nm is desirable for example forreasons of the achievable flatness of the substrates to be structuredand mechanical tolerances.

For this reason, inter alia, optical systems have been developed whichoperate with moderate numerical apertures and achieve the increase inthe resolution capability substantially by means of the short wavelengthof the used electromagnetic radiation from the extreme ultraviolet range(EUV), in particular having operating wavelengths in the range ofbetween 5 nm and 30 nm. In the case of EUV lithography having operatingwavelengths of around 13.5 nm, for example given image-side numericalapertures of NA=0.3 it is theoretically possible to achieve a resolutionof the order of magnitude of 0.03 μm in conjunction with typical depthsof focus of the order of magnitude of approximately 0.15 μm.

Radiation from the extreme ultraviolet range cannot be focused or guidedwith the aid of refractive optical elements, since the short wavelengthsare absorbed by the known optical materials that are transparent athigher wavelengths. Therefore, mirror systems are used for EUVlithography. A mirror (EUV mirror) having a reflective effect withrespect to radiation from the EUV range typically has a substrate, onwhich is applied a multilayer arrangement having a reflective effectwith respect to radiation from the extreme ultraviolet range (EUV) andhaving a large number of layer pairs comprising alternately lowrefractive index and high refractive index layer material. Layer pairsfor EUV mirrors are often constructed with the layer materialcombinations molybdenum/silicon (Mo/Si) or ruthenium/silicon (Ru/Si).

In order to ensure a best possible uniformity of the lithographicimaging, it is generally endeavoured to produce an intensitydistribution that is as uniform as possible in the illumination fieldilluminated by the illumination system. Furthermore, it is normallyendeavoured to approximate the local intensity distribution of theillumination rays that is desired for a specific exposure in the pupilsurface of the illumination system as exactly as possible to the desiredspatial intensity distribution or to minimize deviations from a desiredspatial intensity distribution. These requirements not only have to bemet by the lithographic optical system at the time of its delivery, buthave to be maintained over the entire lifetime of the optical systemwithout significant change. While in the former case possible deviationsare substantially based on design residues and manufacturing faults,changes over the lifetime are often substantially caused by ageingphenomena.

In optical systems for lithography using ultraviolet light from the deepor very deep ultraviolet range (DUV or VUV), non-uniformities thatpossibly arise can generally be compensated for by driveable mechanicalcompensators (cf. e.g. US 2008/113281 A1 or U.S. Pat. No. 7,545,585 B2).

In optical systems for EUV microlithography, such compensators aresignificantly more difficult to realize, inter alia for geometricalreasons. By way of example, a freely accessible intermediate field planewhich is optically conjugate with respect to the object plane of theprojection lens and in which the field homogeneity can be corrected in asimple manner often does not exist. WO 2010/049020 A1 disclosespossibilities for correcting the illumination intensity distribution andthe illumination angle distribution in the illumination field of an EUVillumination system. Other correction devices are disclosed in US2003/0063266 A1, EP 1 349 009 A2, US 2008/0165925 A1 or WO 2009/135576A1.

OBJECTS AND SUMMARY OF THE INVENTION

A problem addressed by the invention is that of providing an EUV mirrorarrangement and an optical system equipped therewith which can be usedin a microlithography projection exposure apparatus, for example, inorder to ensure, over the entire lifetime of the projection exposureapparatus, a high fidelity and stability of the illumination intensityin field and pupil with regard to a predetermined distribution and thusthe lithographic imaging quality.

For solving this problem, the invention provides an EUV mirrorarrangement comprising the features of claim 1. An optical systemcomprising an EUV mirror arrangement comprising the features of claim 16and a method for operating such an optical system comprising thefeatures of claim 21 are furthermore provided.

Advantageous developments are specified in the dependent claims. Thewording of all the claims is incorporated by reference in the content ofthe description.

The EUV mirror arrangement has a multiplicity of mirror elements whichare arranged alongside one another and jointly form a mirror surface ofthe mirror arrangement. In this case, the element mirror surface of amirror element forms a fraction of the total mirror surface. Mirrorelements can be arranged alongside one another for example in rows andcolumns in a manner substantially filling the surface area or completelyfilling the surface area or else mutually at a distance from oneanother. The mirror elements can be mirror elements that can be mountedseparately from one another and, if appropriate, in a manner separatedby interspaces on a carrier structure.

It is also possible for the mirror elements to have a common substrateand for the multilayer arrangement to have continuous layers over theentire useable region. In this case, the electrode arrangement can haveone or a plurality of structured electrodes in order to be able to applyan electric field of predeterminable strength to the regions of anactive layer which are assigned to the individual mirror elementsindependently of one another.

A multilayer arrangement has a multiplicity of layer pairs eachcomprising a layer composed of a relatively high refractive index layermaterial and a layer composed of a (relative thereto) low refractiveindex layer material. Such layer pairs are also referred to as a “doublelayer” or “bilayer”. A layer arrangement having a large number of layerpairs acts in the manner of a “Distributed Bragg Reflector”. In thiscase, the layer arrangement simulates a crystal whose lattice planesleading to Bragg reflection are formed by the layers of the materialhaving the lower real part of the refractive index. The optimum periodthickness of the layer pairs is determined by the Bragg equation for apredetermined wavelength and for a predetermined angle of incidence(range) and is generally between 1 nm and 10 nm.

A layer pair can also have, in addition to the two layers composed ofrelatively high refractive index and relatively low refractive indexmaterial, respectively, one or a plurality of further layers, forexample an interposed barrier layer for reducing interdiffusion betweenadjacent layers.

The multilayer arrangement of a mirror element has at least one activelayer which is arranged between a radiation entrance surface and thesubstrate and which consists of a piezoelectrically active layermaterial. On account of this material property of the active layermaterial, the layer thickness of the active layer can be altered byapplying an electrical voltage. For each active layer provision is madeof an electrode arrangement for generating an electric field acting onthe active layer. It is thereby possible for the active layers of themirror elements to be activated independently of one another asnecessary and thus to be altered with regard to their layer thicknesses.As a consequence thereof, the reflection properties of the EUV mirrorarrangement can be influenced locally differently over the mirrorsurface.

This makes use of the inverse piezoelectric effect, in which the activelayer material deforms reversibly under the action of an electric field.In this case, the crystalline active layer material does not undergo aphase transformation, rather a displacement of positive and negativecharge centroids within the crystal structure of the electricallynon-conductive active layer material merely takes place.

One electrode of the electrode arrangement can be in touching contactwith the active layer. It is also possible to arrange one or a pluralityof electrodes at a distance from the active layer to be influenced, aslong as the electric field can permeate the material-filled ormaterial-free interspace as far as the active layer. Consequently, oneor a plurality of layers of the layer arrangement can also be situatedbetween an electrode and the active layer. In particular, for generatingthe electric field, a voltage can be applied between an outer layer of alayer arrangement remote from the substrate and an inner layer of alayer arrangement near the substrate, wherein a large number of layerpairs are respectively situated between the electrode layers and theactive layer.

In this case, the lateral resolution (spatial resolution) of theinfluence is dependent on the lateral dimensions of the element mirrorsurfaces of the individual mirror elements. Depending on theapplication, lateral dimensions can be e.g. in the range of one or aplurality of millimeters or centimeters. Smaller lateral dimensions,e.g. of between 1 μm and 900 μm, are likewise possible. More than 10 ormore than 100 or more than 1000 mirror elements which can be drivenindependently of one another can be provided in the mirror surface ofthe mirror arrangement. It can also suffice to provide fewer than 10,for example only two or three or four separately driveable mirrorelements. This can be useful e.g. for alignment purposes or calibrationpurposes.

At least one active layer whose layer thickness can be altered in atargeted manner by the electrical driving of the assigned electrodearrangement is integrated into the multilayer arrangement.

There are various possibilities for arranging the at least one activelayer with respect to the layer pairs of the multilayer arrangement.

In some embodiments, the multilayer arrangement has a first layer grouparranged between the radiation entrance surface and the active layer andhaving a first number N1 of layer pairs, and a second layer grouparranged between the active layer and the substrate and having a secondnumber N2 of layer pairs, wherein the numbers N1 and N2 of layer pairsof the first layer group and of the second layer group are selected insuch a way that, for at least one angle of incidence of the radiationimpinging on the radiation entrance surface, the first layer grouptransmits a portion of the incident radiation through the active layerto the second layer group and the radiation reflected by the multilayerarrangement contains a first portion reflected by the first layer groupand a second portion reflected by the second layer group.

In general, the first and second layer groups in each case have aplurality of layer pairs, e.g. in each case 10 or more, or 15 or more,layer pairs.

In this case, both the first layer group remote from the substrate andthe second layer group near the substrate contribute to the totalreflectivity of a mirror element. By means of the interposed activelayer, the distance between the layer groups (measured perpendicularlyto the layer surface) can be altered by applying an external voltage.The layer construction of the first layer group is preferably chosensuch that, for the angle of incidence or angle-of-incidence rangeconsidered, constructive interference of the portions of radiation(partial waves), reflected at the individual interfaces within the firstlayer group occurs. The same correspondingly preferably applies to thelayers of the second layer group as well. The interposed active layerintroduces an optical path length difference or a phase shift betweenthe portions of radiation reflected at the first layer group and theportions of radiation reflected at the second layer group. By applyingan external voltage, it is possible for the extent of the phase shift tobe varied in a continuously variable manner.

If, by way of example, the phase shift introduced, in the absence of anelectric field, is substantially one wavelength or an integral multipleof the wavelength of the electromagnetic radiation, then the firstportion and the second portion of the reflected radiation interfereconstructively with one another, such that the total reflectivity of themirror element can lie in the range of the maximum possible reflectivityapplicable to the angle-of-incidence range. By contrast, if the layerthickness of the active layer is set such that the phase shift betweenthe first and second portions is in the range of one half wavelength orin the range of three half wavelengths, etc., then destructiveinterference takes place between the first portion and the secondportion, such that the total reflectivity resulting from the firstportion and the second portion is lower than the maximum reflectivitymaximally possible with the layer groups.

If, by way of example, the change in the optical path length upon singlepassage through the active layer is one quarter of the operatingwavelength, and if the active layer is positioned at a suitable depth insuch a way that the first and second portions have substantially thesame intensity, then the reflection can be substantially completelysuppressed. Between these extremes (maximum reflectivity of a mirrorelement and complete suppression of the reflection of a mirror element)numerous variants arise which will be explained in greater detail inconnection with the exemplary embodiments.

The active layer integrated between the first and second layer groupsacts in the manner of an integrated Fabry-Perot interferometer (etalons)with an electrically adjustable distance between its interfaces having areflective effect.

In many cases it is not necessary or required to vary the reflectivityof a mirror element between maximum reflection and complete suppressionof reflection. It often suffices if the degree of reflection of a mirrorelement is varied only by a maximum of 20% or a maximum of 10%. In someembodiments, the active layer, in the absence of an electric field, hasa layer thickness chosen in such a way that for a reference angle ofincidence of the incident radiation a reflectivity of the multilayerarrangement can be altered by a maximum of 20%, in particular a maximumof 10%, by applying an electric field.

Preferably, exactly one active layer is provided between two adjacentlayer groups each having a plurality of layer pairs. It is therebypossible, inter alia, for the risk of incorrect coating on account ofmanufacturing tolerances to be kept small. Moreover, this results inonly a low complexity between the transmitted and the absorbed portionsof radiation. However, a multilayer arrangement can also have more thanone active layer which is arranged between two adjacent layer groupshaving a plurality of layer pairs and which serves for the controllablephase shift between the reflected portions of radiation of these layergroups. By way of example, two or three of such active layers can beprovided, between which layer groups having a plurality of layer pairsare then likewise situated.

In the selection of active layer materials for such an integrated activelayer, consideration should be given to ensuring that the layer materialon the one hand has only relatively low absorption for the radiation tobe transmitted to the second layer group and on the other hand enables asufficiently great “swing” of the layer thickness for the control of thephase shift. In some embodiments, the active layer materialsubstantially consists of barium titanate (BaTiO₃).

Generally, for the piezoelectrically active layer preference is given tolayer materials which, in the EUV wavelength range chosen, have arelatively low absorption (low extinction coefficient or imaginary partof the complex refractive index) and at the same time exhibit arelatively great piezoelectric effect in order to be able to producesufficiently great changes in layer thickness. The piezoelectricallyactive layer material can be a material having a perovskite structurewhich exhibits a relatively great piezoelectric effect. In particular,the piezoelectrically active layer material can be selected from thegroup: Ba(Sr,Zr)TiO₃, Bi(Al,Fe)O₃, (Bi,Ga)O₃, (Bi,Sc)O₃, CdS, (Li,Na,K)(Nb,Ta)O₃, Pb(Cd,Co,Fe,In,Mg,Ni,Sc,Yb,Zn,Zr) (Nb,W,Ta,Ti)O₃, ZnO, ZnSor contain at least one material of this group in combination with atleast one other material. In this case, the notation (A,B) denotes thatan element or ion of the type A or an element or ion of type B can bepresent in a specific lattice position of the crystal structure.

In other embodiments, the multilayer arrangement has a multiplicity ofactive layers composed of a piezoelectrically active layer material,wherein the active layers are respectively arranged alternatively withlayers composed of a non-piezoelectrically active layer material. Inthis case, the layers arranged between the active layers preferablyconsist of an electrically conductive layer material, such that theselayers can simultaneously serve as electrode layers for the activelayers respectively arranged therebetween. The active layer material canbe, in comparison with the non-active layer material, either therelatively high refractive index or the relatively low refractive indexlayer material. An active layer material having relatively highabsorption can advantageously be used as an absorber layer.

In this configuration, by applying an electric field to the activelayers, it is possible to produce an, if appropriate continuouslyvariable, variation of the layer period within the multilayerarrangement. In this case, the layer period denotes the distancemeasured perpendicularly to the layer surface between the bounding outerinterfaces of a layer pair. Since, for a given operating wavelength anda given angle of incidence, only specific layer periods lead to fullconstructive interference and hence to a maximum degree of reflection,by varying the layer period it is possible to alter the reflectivity ofthe multilayer arrangement of the mirror element at the operatingwavelength in a continuously variable manner. Furthermore, the phase ofthe reflected radiation is influenced, such that a spatially resolvingwavefront influencing is also possible.

The detuning or changing of the layer period can also be used to adaptthe reflectivity to a central wavelength possibly deviating from thedesired value, such that e.g. a compensation of variations of the sourcespectrum or of the spectral transmission of the overall optical systemcan be carried out. Alternatively or additionally, adaption to desirablyor undesirably altered angles of incidence on the mirror is alsopossible.

In embodiments comprising a multiplicity of active layers composed of apiezoelectrically active layer material and arranged alternatively withnon-active layers composed of a non-piezoelectrically active layermaterial, it is also possible to generate a non-periodic layerstructure. To this end, the individual non-active layers formed byelectrically conductive, non-piezoelectrically active layer material canbe connected to individual outputs of a voltage source so that the fieldstrength of electric field affecting the different active layers can beset differently, if required. Active layers with differing thickness canbe obtained depending on the voltages applied to each pair ofelectrodes. A broadband spectral reflectivity response may be obtained.

In multilayer arrangements having a multiplicity of active layers,consideration should be given particularly to ensuring that the activelayer material has low absorptions for the used radiation. In thisconnection it has proved to be advantageous if the active layer materialpredominantly or exclusively consists of a ceramic material of the type(Li, Na, K)(Nb, Ti)O₃. Such materials are described, e.g. in EP 2 050726 A2. These materials can also be advantageous from health aspects,since they do not contain lead (Pb).

In particular, the active layer material can contain or consist of amaterial from the group potassium niobate (KNbO₃), lithium niobate(LiNbO₃), PbNb₂O₆ and sodium potassium niobate (Na_(0.9)K_(0.1)NbO₃) orof a combination of these materials. These materials are distinguished,inter alia, by particularly low absorption in the EUV range.

It is also possible to design the EUV mirror arrangement such that,substantially without influencing the local distribution of thereflectivity, a spatially resolving phase correction of the wavefront ofthe impinging radiation is possible. Such embodiments can be used, inparticular, as a mirror in an EUV projection lens. In some embodimentsof this type, the multilayer arrangement has a third layer grouparranged between the radiation entrance surface and the active layer andhaving a third number N3 of layer pairs, wherein the third number N3 isselected in such a way that, for at least one angle of incidents of theradiation impinging on the radiation entrance surface, the third layergroup substantially completely reflects or absorbs the incidentradiation before the latter reaches the active layer. By way of example,at least 20 or at least 30 or at least 40 layer pairs can be provided.Typically, there are fewer than 70 or fewer than 60 layer pairs.

In this case, the reflectivity (or the degree of reflection) of themirror element is practically exclusively determined by the layerconstruction of the third layer group. This can be raised or loweredwith the aid of the active layer with respect to the substrate byapplying an electrical voltage without tilting perpendicularly to thelayer surface as a whole.

The layer thicknesses of the individual layers of the layer pairs and,if appropriate, also of the active layer are generally of the order ofmagnitude of a few manometers. In order to minimize the influence ofinterface roughnesses on the optical effect of the mirror elements, inpreferred embodiments provision is made for applying the active layerand/or possible electrode layers with the aid of pulsed laser deposition(PLD), such that the active layer and/or an electrode layer are/ispresent as a PLD layer. With the aid of pulsed laser deposition it ispossible to produce very thin layers having little surface roughness. Asnecessary, it is also possible to produce monocrystalline piezoelectriclayer materials having a high piezoelectric coefficient, the surface ofwhich can then be used without polishing as a contact surface forfurther layers.

Preferably, at least that layer to which the active layer is applied isproduced as a crystalline (non-amorphous) layer, in particular with theaid of pulsed laser deposition. This facilitates crystal growth of theactive layer. The latter can, in favourable cases, grow epitaxially withrespect to an underlying crystalline layer. In some embodiments, most orall of the layers lying between the substrate and active layer arecrystalline.

In some embodiments, the electrode arrangement of a mirror element has afirst electrode layer and a second electrode layer and the active layeris arranged between the electrode layers. What can thereby be achievedis that the electric field permeates the active layer substantiallyperpendicularly to the layer surface, as a result of which the changesin layer thickness can be produced particularly effectively. Anelectrode layer can consist of a metallic layer material or of asemimetal, such as silicon, for example. Electrode layers composed ofsilicon can be arranged for example alternately with active layerscomposed of a piezoelectrically active layer material.

An electrode layer may consist of a single layer or may comprise aplurality of single layers stacked on top of each other to form a layerstack (or multilayer).

In some embodiments comprising a laterally structured layer electrode,the electrode arrangement comprises a common electrode opposite to thestructured layer electrode, the common electrode extending over aplurality of mirror elements or all mirror elements. When connected to avoltage source the common electrode may serve as a common referencepotential for each of the electrode segments arranged on the other sideof the active layer opposite thereto. The electrode segments may be setto different voltage values to adjust the thickness of the active layerin a locally varying manner.

The common electrode may be formed on the substrate side of the activelayer. In some embodiments the common electrode is formed on theradiation incidence side of the active layer, i.e. opposite to thesubstrate. In this case it may be advantageous if the common electrodecomprises a plurality of single layers stacked on top of each other toform a layer stack, or multilayer. A smooth uninterrupted reflectivesurface of the mirror arrangement may be obtained.

In some cases it has proved to be advantageous if an electrode layerconsists of an electrically conductive ceramic material, for example ofSrRuO₃ or aluminium nitride (AlN). The use of electrically conductiveceramic materials as electrode material, in conjunction with ceramicactive layer materials, allows the lattice mismatch at the interfacesbetween electrode layer and active layer material to be kept small, as aresult of which layer stresses in the region of the interfaces and hencethe risk of layer detachment can be kept low and, consequently, thelifetime of the layer arrangement can be improved.

The invention also relates to an optical system comprising at least oneEUV mirror arrangement. The optical system can be, in particular, anillumination system of a microlithography projection exposure apparatus.The EUV mirror arrangement can be arranged in the beam path of theillumination system between a light source and an illumination field tobe illuminated in or near a field plane which is situated in a manneroptically conjugate with respect to a plane of the illumination field.In this case, the EUV mirror arrangement can serve as a field facetmirror. Alternatively or additionally, an EUV mirror arrangement can bearranged in the region of a pupil plane of the illumination system, thatis to say in the region of a plane which is situated in the mannerFourier-transformed with respect to the plane of the illumination field.In this case, the EUV mirror arrangement can serve as a pupil facetmirror. The optical system can also be a projection lens of amicrolithography projection exposure apparatus.

In a method for operating an optical system comprising at least one EUVmirror arrangement of this type, the local reflectivity distributionover the mirror surface of the EUV mirror arrangement can be varied in alocation-dependent manner by selectively driving individual or all ofthe active layers. If an EUV mirror arrangement is in this case arrangedin the region of a field plane of the optical system, it is therebypossible to influence the illumination intensity distribution in saidfield plane and in field planes that are optically conjugate withrespect thereto. In the case of an arrangement in the region of a pupilplane, by locally changing the reflectivities it is possible to alterthe illumination intensity distribution in the illumination field in anangle-dependent manner.

These and further features emerge not only from the claims but also fromthe description and the drawings, wherein the individual features can berealized in each case by themselves or as a plurality in the form ofsubcombinations in an embodiment of the invention and in other fieldsand can constitute advantageous and inherently protectable embodiments.Exemplary embodiments are illustrated in the drawings and are explainedin greater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic, obliquely perspective illustration of anembodiment of an EUV mirror arrangement in partial section;

FIG. 2 shows a diagram for elucidating the influence of the layerthickness of an active layer on the reflectivity R and the transmittanceT of the entire layer arrangement;

FIG. 3 shows a diagram concerning the influence of the number of layerpairs in the first layer group remote from the substrate on thereflectivity profile as the layer thickness of the active layerincreases;

FIG. 4A shows a diagram concerning the reflectivity profile in theregion of a reflectivity maximum with an increasing number of layerpairs in the second layer group closest to the substrate, with aconstant number of layer pairs in the first layer group;

FIG. 4B shows a diagram concerning the reflectivity profile in theregion of a first reflectivity maximum;

FIG. 5 shows a diagram of the reflectivity profile in the region of areflection maximum, in the case of which it is possible to obtain asetting range ΔR of the reflectivity of approximately 2.5% given a layerthickness variation Δz of 0.127 nm;

FIG. 6 shows a diagram with reflectivity and transmission profiles of amultilayer arrangement which can be used as a beam splitter withtransmission that can be set in a variable manner;

FIG. 7 schematically shows part of an EUV layer arrangement with anactivated and a non-activated mirror element;

FIG. 8 shows a schematic, obliquely perspective illustration of afurther embodiment of an EUV mirror arrangement in partial section;

FIG. 9 shows a schematic, obliquely perspective illustration of afurther embodiment of an EUV mirror arrangement in partial section;

FIG. 10 shows different embodiments of structured layer electrodes inFIGS. 10A, 10B and 10C;

FIG. 11 shows optical components of an EUV microlithography projectionexposure apparatus with embodiments of EUV mirror arrangements which areused as a field facet mirror and as a pupil facet mirror, respectively;

FIG. 12 shows an embodiment comprising a structured electrode comprisingmultiple multi-layered electrode segments on the substrate-side of anactive layer and a continuous electrode extending across multiple mirrorelement on a radiation incidence side of the active layer; and

FIG. 13 shows an embodiment comprising multiple active layers which canbe controlled individually.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a schematic, obliquely perspective illustration of anembodiment of an EUV mirror arrangement 100 in partial section. Themirror arrangement has a multiplicity of mirror elements 110, 111, 112which are arranged alongside one another and which each have arectangular cross section in the case of the example. Each mirrorelement can be designated as an individual mirror and has a rectangularelement mirror surface, wherein the element mirror surfaces adjoin oneanother largely without any gaps or lie aside one another withinterspaces and jointly form a mirror surface 115 of the mirrorarrangement. The mirror surface can be flat (plane mirror) or curved(e.g. convex mirror, concave mirror, cylindrical mirror, etc.) overall.

The construction of a mirror element will be explained in greater detailon the basis of the mirror element 110. Each mirror element has asubstrate 120, which can consist, for example, of metal, silicon, aglass, a ceramic material, a glass ceramic or a composite material. On asubstrate surface processed in a smooth fashion with high precision, amultilayer arrangement 130 having a reflective effect with respect toradiation from the extreme ultraviolet range is applied by suitablecoating technology. For producing some or all of the individual layers,it is possible to use e.g. magnetron sputtering, electron beamsputtering or ion beam sputtering. If a crystalline layer structure isdesired, it is also possible to effect coating by means of pulsed layerdeposition (PLD), for example.

The multilayer arrangement has a multiplicity of layer pairs (bilayers)135 each having alternately applied layers of a layer material having ahigher real part of the refractive index (also called “spacer”) and alayer material having relative thereto a lower real part of therefractive index (also called “absorber”). In the case of the example,relatively thin layers 136 comprising molybdenum (Mo) as absorbermaterial are applied alternately with relative thereto thicker layers134 comprising silicon (Si) as spacer material. A layer pair can alsocontain at least one further layer, in particular an interposed barrierlayer, which can consist e.g. of C, B₄C, Si_(x)N_(y), SiC or acomposition comprising one of said materials and is intended to preventinterdiffusion at the interface. It is thereby possible to ensurepermanently sharply defined interfaces including under radiationloading.

The layer pairs are grouped into two layer groups. A first layer group131 remote from the substrate and near the surface has a first number N1of layer pairs 135. In the case of the example, a cap layer 137 forprotecting the underlying layers is also applied between the first layergroup and the radiation entrance surface remote from the substrate. Thecap layer can consist e.g. of ruthenium, rhodium, gold, palladium,Si_(x)N_(y) or SiC or contain one of said materials. The free surface ofthe cap layer forms the radiation entrance surface.

A second layer group 132 near the substrate has a second number N2 oflayer pairs 135. This second layer group can be applied directly to thesubstrate surface, but it is also possible to provide a single- ormultilayered intermediate layer functioning as a smoothing layer, forexample. The layers of the second layer group near the substrate arepreferably produced as crystalline layers, in particular by means ofpulsed laser deposition (PLD).

An individual active layer 140 composed of a piezoelectrically activelayer material is arranged between the first layer group 131 and thesecond layer group 132. The layer thickness z of the active layer can bealtered by applying an electric field to the active layer material. Forthis purpose, in direct contact with the active layer, a first electrodelayer 142 is arranged between said active layer and the first layergroup and a second electrode layer 143 is arranged between the activelayer and the second layer group. The electrode layers in areal contactwith the active layer material consist of an electrically conductivelayer material and are connected by electrically conductive connectionsto a switchable or regulable voltage source 145. The layer thickness zcan be varied, in a manner dependent on the voltage generated by thevoltage source, continuously variably between a minimum value z_(min)(in the absence of an electric field) and a maximum value z_(max).

A corresponding electrode arrangement is provided for each of the mirrorelements. The electrode arrangements can be driven independently of oneanother, such that, for each mirror element, the active layer thereofcan be altered with regard to its layer thickness by applying electricalvoltage independently of the active layers of other mirror elements.

Both the first layer group 131 and the second layer group 132 aredesigned, with regard to the layer thicknesses of the spacer andabsorber layers, such that they have a reflective effect with respect tothe angle-of-incidence range with which the mirror arrangement isintended to be operated. In this case, the first number N1 of layerpairs of the first layer group 131 near the surface is chosen such thatsaid layer group reflects only part of the radiation incident at themirror surface by Bragg reflection and another portion of the incidentradiation is transmitted through the active layer 140 to the secondlayer group 132. The second number N2 of layer pairs of the second layergroup is chosen such that said portion transmitted to the second layergroup is practically completely reflected (and, if appropriate, partlyabsorbed) by the second layer group.

The portion reflected by the second layer group 132 is reflected backthrough the active layer and through the first layer group. Accordingly,the radiation reflected overall by the layer arrangement contains afirst portion reflected by the first layer group and a second portionreflected by the second layer group. The resultant total reflectivity ofthe multilayer arrangement (that is to say the ratio between reflectedand incident intensity, represented by the degree of reflection orreflectivity R) is in this case determined by interference between thepartial waves reflected by the first layer group 131 and the partialwaves reflected by the second layer group 132. In this case, the typeand extent of the interference can be altered separately for each mirrorelement by altering the layer thickness of the active layer, such thatwithin predetermined setting limits it is possible to alter theweighting between portions of destructive interference and constructiveinterference. In this case, the extent of interference is determined bythe optical angular length difference (phase difference), between thepartial waves reflected by the lower second layer group 132 and thepartial waves reflected by the upper first layer group 131. This basicprinciple will be explained in greater detail below on the basis ofcalculated exemplary embodiments.

In the calculated exemplary embodiment, barium titanate (BaTiO₃) is usedas active layer material for the active layer 140. The first electrode142 and the second electrode 143 are respectively formed by layerscomposed of SrRuO₃. This electrically conductive ceramic materialexhibits relatively little lattice mismatch in contact with bariumtitanate. Alternatively, for example, aluminium nitride (AlN) or someother electrically conductive material, for example a metallic material,can be used. The layer pairs 135 consist, as mentioned, of molybdenum asabsorber material and silicon as spacer material.

At least the lower electrode layer 143 on the substrate side is presentas crystalline layers; it can be produced by means of pulsed laserdeposition (PLD), in particular. The crystal surface then serves as asupport for the growth of the active layer, which is likewise acrystalline layer applied by means of PLD. On account of the smalllattice mismatch, if appropriate epitaxial growth of a monocrystallineactive layer on a monocrystalline lower electrode layer is possible.

Depending on the layer thickness of the active layer and the coatingmethod used for producing it, its surface can be relatively rough. Inorder to improve the growth conditions for subsequent layers, it ispossible to apply to the active layer a smoothing layer, e.g. composedof amorphous silicon, the surface of which can then be polished smoothby means of an ion beam before the next layer is applied. Said smoothinglayer can serve as an electrode layer.

In order to demonstrate the effect of the layer thickness z of theactive material, examples are presented below which were calculated foran operating wavelength λ=13.5 nm of the EUV radiation and normalradiation incidence (angle of incidence AOI=0°). In this case, the term“angle of incidence” denotes the angle between the direction ofincidence of a ray and the normal to the surface of the mirror at thepoint where the ray impinges on the mirror. The starting structure shallconsist of 10 Mo/Si layer pairs in the first layer group 131 (i.e.N1=10) and likewise 10 Mo/Si layer pairs in the second layer group 132(i.e. N2=10). Given a layer thickness of the Mo layers of 2.76 nm and4.14 nm for the Si layers, this results in a periodicity length d=6.9 nmfor the stack of layer pairs. The layer thickness of the electrodelayers 142, 143 is 2.76 nm in each case. The layer thickness z of theactive layer is variable.

For elucidating the influence of the layer thickness on the reflectivityR and the transmittance T of the entire layer arrangement, FIG. 2 showsa corresponding diagram illustrating the reflectivity R and thetransmittance T as a function of the layer thickness z [nm] for theabove example. Depending on the layer thickness z and the phasedifference caused thereby between the portions of radiation reflected bythe layer groups, periodic changes result as the layer thickness zincreases, thus giving rise to maxima (peaks) and minima (valleys) inthe curves, wherein the maxima of the transmission T naturally lie inthe region of the minima of the reflectance R.

The layer thickness z can now be defined with respect to various targetstipulations. In the first region R1 bounded by horizontal dashed lines,in the case of layer thicknesses of around 5 nm and around 12 nm thereare respectively regions with maximum reflectivity R. In these regions,on account of the small gradient of the reflectivity curve, for a givenlayer thickness variation Δz, only a relatively small setting range(tuning range) arises for the variation of the total reflectivity. Thesecond region R2 lying underneath covers the regions with a relativelyhigh gradient of the reflectivity curves respectively on the left andright of the reflectivity maxima. For a given layer thickness variationΔz of the active layer, here particularly large setting ranges ΔR arisefor the total reflectivity (cf. FIG. 5), but the absolute value of thereflectivity is somewhat smaller than in the regions of maximumreflectivity (region R1). The third region R3 marks the regions ofminimum reflectivity and corresponding maximum transmission of the layerarrangement. If corresponding layer thicknesses of the active layer arechosen, the layer arrangement can also be used as a beam splitter layerhaving a settable ratio between reflectivity and transmission.

The influence of the number of layer pairs in the first layer group 131remote from the substrate on the reflectivity profile as the layerthickness z increases will now be elucidated with reference to FIG. 3.In this respect, FIG. 3 shows the region of the first reflectivitymaximum (in the case of layer thicknesses z of around 5 nm), wherein thesecond number N2=10 remains constant and the first number N1 of layerpairs in the first layer group near the surface varies between N1=10 andN1=25. It can be discerned that the maximum reflectivity in the regionof the first reflectivity maximum increases from approximately 0.6 toapproximately 0.72 as N1 increases, and that the indentations in theregion of the adjacent reflectivity minima become shallower, such thatthe reflectivity varies over the layer thickness for N1=25 only betweenR=0.6 and R=0.72.

It is demonstrated with reference to FIG. 4A how the reflectivityprofile in the region of the reflectivity maximum behaves with anincreasing number of the layer pairs in the second layer group 132closest to the substrate given a constant number N1=10 of the layerpairs in the first layer group. With an increasing number of layer pairsin the second layer group closest to the substrate, the maximumreflectivity increases from approximately 0.6 to approximately 0.7 inthe region of the reflectivity maximum, while the decrease in thereflectivity in the region of the adjoining reflectivity minima (atapproximately z=1.5 nm and z=8.4 nm) increases.

It is evident from this that there are many pairings of first and secondlayer groups (corresponding to first numbers N1 and second numbers N2)which produce a high reflectivity in the vicinity of the absolutereflectivity maximum. On the basis of these curves it is possible toselect pairings of first and second numbers N1 and N2, respectively,which allow a large setting range with a varying layer thickness z ofthe active layer in conjunction with relatively high total reflectivity.

It should be noted that the maximum reflectivity cannot be increasedarbitrarily by a larger number of layer pairs. Rather, e.g. in the caseof Mo/Si layer pairs, experience shows that saturation arises atapproximately 50 layer pairs. In the exemplary calculations, the maximumnumber of layer pairs (N1+N2) was limited to 48, since higher numbers oflayers hardly entail any significant changes to the overall behaviour.

The setting range (tuning range) is primarily determined by theelasticity and the yield stress of the piezoelectrically active layermaterial. When the yield stress (π_(y)) is exceeded, an irreversibledeformation of the layer material commences. The yield stress is linkedto the elasticity (described by the modulus of elasticity E, also calledYoung's modulus) of the material and the dimensional change ordeformation of the material, for which the strain ε serves as anormalized measure. The relationship between the dimensional change(strain) of the layer thickness which is possible without plasticdeformation of the material (ε_(max)=Δz/z), the yield stress and themodulus of elasticity is given by ε_(max)=σ_(y)/E. In this case, z isthe initial layer thickness and Δz is the change in layer thickness. Theyield stress for piezoelectric materials is typically between 1% and 5%and is approximately 4.8% for BaTiO₃ (R. F. Cook, C. J. Fairbanks, B. R.Lawn and Y.-W. Mai “Crack Resistance by Interfacial Bridging: Its Rolein Determining Strength Characteristics,” J. Mater Res., 2, 345-356(1987)).

Furthermore, the layer thickness of the active layer prior to layerexpansion shall be described by z_(min) and the change in layerthickness by Δz. Depending on which side of a reflectivity maximum istaken as a basis for considering the optimization process, the thicknessof the piezoelectric material at the minimum reflectivity of the settingrange and the maximum reflectivity of the setting range shall be givenby z_(max)=z_(min)+Δz. Using this information and the yield stress ofBaTiO₃ (σ_(y)=Δz/z_(min)), it is possible to calculate z_(min), z_(max)and Δz. If, by way of example, the desired maximum reflectance is set toR_(max)=72% and the number of layer pairs (N1 or N2) is limited to 48,the values for the first five reflection maxima (peak 1 to peak 5)indicated in Table 1 are obtained. In this case, N1=N₁ and N2=N₂ holdtrue.

The values z_(min), z_(max) and Δz are indicated in Tables 1, 2 and 3respectively in the unit 10⁻¹⁰ m or 0.1 nm (corresponding to theconventional, but no longer generally permissible length unit Å(Ångstrom)).

One appropriate solution in the region of the first reflection maximum(in the case of layer thicknesses z of approximately 5 nm) is shown inFIG. 4B, where here R_(max)=72%, N1=16 and N2=16.

TABLE 1 N₁ N₂ R_(max) R_(min) ΔR z_(max) z_(min) Δz Peak 1 16 16 72.045571.9528 0.0927 45.50 43.4160 2.0840 Peak 2 16 36 72.0044 71.5439 0.4605115.25 109.0714 5.2736 Peak 3 19 25 72.0210 71.0067 1.0123 185.75177.2424 8.5076 Peak 4 21 48 72.4302 70.8417 1.5886 255.50 243.797711.7023 Peak 5 22 48 72.2566 69.4827 2.7739 325.25 310.3531 14.8969

If a layer construction is intended to be optimized with regard to amaximum setting range of the reflectivity in conjunction with arelatively high reflectance, the range in the second region R2 in FIG. 2is preferably employed. In this range it is possible to achieve largesetting ranges in conjunction with relatively small dimensional changes(changes in layer thickness) of the active layer, the maximumreflectance not quite being achievable there. However, if, for example,the minimum reflectivity is limited to 65%, then it is possible, forexample, to obtain a particularly large setting range with N1=18 andN2=28 in the region of the first reflection maximum. The results of anoptimization in the region of the first reflection maxima are compiledin Table 2.

TABLE 2 N₁ N₂ R_(max) R_(min) ΔR z_(max) z_(min) Δz Peak 1 18 28 67.571365.0722 2.4992 27.7720 26.5 1.2720 Peak 2 28 48 71.0973 65.1657 5.931794.3200 90 4.3200 Peak 3 25 48 72.0965 65.1191 6.9774 169.2520 161.57.7520 Peak 4 24 29 72.1154 65.0037 7.1117 242.6120 231.5 11.1120 Peak 524 32 72.3050 65.0045 7.3005 314.9240 300.5 14.4240

It can be discerned, inter alia, that larger absolute layer thicknessesof the active layer (corresponding to the second, third, fourth, etc.reflectivity maximum), enable a larger layer thickness swing Δz andhence a larger setting range ΔR of the reflectivity. It is possible tochoose a suitable compromise with regard to absorption by the activelayer.

FIG. 5 shows the reflectivity profile in the region of the firstreflection maximum for N1=18 and N2=28. It can be discerned from Table 2that in this region it is possible to obtain a setting range ΔR of thereflectivity of approximately 2.5% given a layer thickness variation Δzof 0.127 nm.

In one embodiment, in particular at least one of the followingconditions can hold true:10<N1<30  (1)15<N2<50  (2)30<(N1+N2)<70 and N1>10 and N2>10  (3)N1≤N2  (4)z≥2 nm  (5)z≤35 nm  (6)Δz≥0.1 nm  (7)0.15 nm≤Δz≤2 nm  (8)

Finally, it shall also be explained with reference to FIG. 6 thatmultilayer arrangements of the type described can also be used as aphysical beam splitter having a transmission that can be set variably.The third region R3 from FIG. 1 having relatively large values of thetransmission is particularly suitable for this application. FIG. 6shows, by way of example, the region around the second localtransmission maximum (corresponding to the first reflectivity minimum),in the case of layer thicknesses z≠1.2 nm. Here too, the calculation wascarried out for an angle of incidence AOI=0°. Corresponding resultswould also arise for the angle-of-incidence range around 45°, which isbetter suited to the application. In the range considered, forcalculation purposes the minimum transmission was set to 30% and thetransmittance was varied above this level. Table 3 indicates exemplaryvalues for N1=8 and N2=8. In this case, Tzmin and Rzmin are thetransmission and reflectivity, respectively, at the minimum layerthickness zmin, and Tzmax and Rzmax are the corresponding values at themaximum layer thickness zmax.

TABLE 3 Tzmin 30.3426 Rzmin 28.4563 zmax 94.5 Tzmax 36.1263 Rzmax17.2831 zmin 90.1718 ΔT 5.7837 ΔT 11.1732 Δz 4.3282

The functioning of an EUV mirror arrangement 700 is shown schematicallywith reference to FIG. 7, said EUV mirror arrangement comprising aplurality of mirror elements whose multilayer arrangement in each casehas an integrated piezoelectrically active layer 140 between a firstlayer group 131 near the surface and a second layer group 132 near thesubstrate. The periodicity of the successive layer pairs shall in eachcase be adapted to the used angle of incidence AOI such that a maximumreflectivity respectively arises both within the first layer group andwithin the second layer group. Furthermore, the layer thickness z of theactive layer 140, in the absence of an electric field, shall bedimensioned such that full constructive interference arises between thesecond portion A2 originating from the second layer group and the firstportion A1 originating from the first layer group. In the mirror elementshown on the left in the case of the example, this leads to a totalreflectivity in the vicinity of a reflectivity maximum (see diagramabove). The resulting intensity of the reflected radiation isrepresented by the arrow length of the rays emerging from the mirrorsurface.

If the intention is then to set a location-dependent variation of thereflectivity over the entire mirror surface, electrical voltages ofdifferent magnitudes can be applied to the active layers of theindividual mirror elements, such that different layer thicknesses of theactive layers are established within the individual mirror elements. Inthe case of the example, by applying an electrical voltage to the activelayer of the right-hand mirror element, this results in an increase inthe layer thickness by Δz. This then leads, by comparison with the caseof full constructive interference (left), to a phase shift between thepartial waves originating from the first layer group (portion A1) andthe partial waves originating from the second layer group (portion A2)in such a way that partial destructive interference occurs. As a result,the reflected total intensity decreases by ΔR in the case of the example(see diagram above), which is illustrated by the relatively shorterarrows of the emerging rays.

It should be mentioned that each mirror element can have a differentoperating point or a different nominal layer thickness of the activelayer. Layer elements can also have groupwise identical layerthicknesses which differ between two or more groups.

FIG. 8 schematically shows an EUV mirror arrangement 800 in accordancewith a further embodiment in oblique perspective and partial section.This EUV mirror arrangement also has a large number of mirror elements810, 811, 812 which are arranged alongside one another in rows andcolumns such that their individual element mirror surfaces overall formthe total mirror surface of the mirror arrangement. The layerconstruction of the mirror element 810 is explained in greater detail byway of example. A multilayer arrangement 830 is applied on a substrate820 by means of suitable coating techniques. Over the majority of itsthickness, the multilayer arrangement has a strictly periodicconstruction comprising a multiplicity of layer pairs 835 wherein eachlayer pair has a relatively thin layer 836 composed of a layer materialhaving a relatively low real part of the refractive index and a thickerlayer 840 composed of a layer material having a relatively higher realpart of the refractive index.

The thicker layers 840 in each case consist of the same,piezoelectrically active layer material and thus form an active layerwhose layer thickness can be altered by the action of an electric field.The thinner layers 836 in each case consist of an electricallyconductive material. Respectively adjacent layers 836 enclose anindividual active layer 840 and serve as electrode layers for the activelayer situated therebetween, in order to generate an electric fieldwhere an electrical voltage is applied between the adjacent layers 836,said electric field permeating the active layer 840 situatedtherebetween perpendicularly to the layer surface. The electrode layers836 are alternately connected to respective poles of a switchable DCvoltage source 845 having a voltage that can be set variably.

The periodic arrangement formed by the sequence of electricallynon-conductive active layers 840 and electrically conductive electrodelayers 836 has a periodicity length P corresponding, in the case of theexample, to the layer thickness of a layer pair 835. The periodicitylength can be altered in a continuously variable manner by applying anelectrical voltage to the electrode layers, since the layer thickness zof the active layers changes in a manner dependent on the appliedvoltage.

As a result of the variation of the layer period P as a reaction to theapplication of an electrical voltage, it is possible to influence thereflectivity of the affected mirror element at the operating wavelength.In a manner coupled by the Bragg equation, it is also possible tocompensate for changes in reflectivity by slightly detuned operatingwavelengths and/or angles of incidence. Since, moreover, the absolutethickness of the entire layer stack, that is to say the distance betweenthe substrate and the element mirror surface of the individual mirrorelement, increases in the case of an increase in the layer thicknessesof the active layers, the wavefront of the radiation impinging on themirror surface is also influenced, since, by way of example, upon theelement mirror surface being raised, the optical path of the radiationreflected by the element mirror surface is shortened overall. A phaseshift can thereby be introduced relative to adjacent non-activatedelement mirror surfaces or element mirror surfaces raised to a differentextent. This embodiment can therefore be used simultaneously for thespatially resolving influencing of the wavefront and for the spatiallyresolving influencing of the reflectivity.

Since, in this embodiment, overall relatively large thicknesses of theactive layer material are traversed by radiation, a material having alow absorption coefficient (imaginary part of the complex refractiveindex) should be used as active layer material.

The number of layer pairs 835 can be between 10 and 70, for example.

In the embodiment of an EUV mirror arrangement 900 in FIG. 9, mirrorelements 910, 911, 912 are likewise arranged alongside one another inrows and columns substantially in a manner filling the surface area suchthat their rectangular element mirror surfaces overall form the mirrorsurface of the mirror arrangement. The layer construction of themultilayer arrangement 930, which is reflective to the EUV radiation, isdesigned such that the EUV mirror arrangement as a whole can be used asa wavefront correction device that is effective in a spatially resolvingfashion, without different spatial distributions of the reflectivityarising for the different operating modes. For this purpose, themultilayer arrangement has, proceeding from the mirror surface, firstlya third layer group 933, which consists of a third number N3 of layerpairs 935 of identical type. Each layer pair (bilayer) has a thinnerabsorber layer 936 composed of molybdenum and a thicker spacer layer 934composed of silicon. The number N3 of layer pairs 935 is chosen suchthat the periodic stack reflects the entire radiation incident from theradiation entrance surface (or else absorbs a relatively small portionthereof). For this purpose, by way of example, between 40 and 50 layerpairs 935 can be provided. The period of the layer pairs is chosen in amanner dependent on the angle-of-incidence range that occurs and theoperating wavelength such that maximum or almost maximum reflectivityoccurs in accordance with the Bragg equation.

An active layer 940 composed of a piezoelectric active layer material issituated between the third layer group 933 and the substrate 920.Electrode layers 942 and 943 are respectively arranged at the top sideand the underside of the active layer, wherein the electrode layer 943near the substrate can be arranged directly on the substrate 920 or, inother embodiments, on an intermediate layer situated in between. Theelectrode layers 942, 943 are connected to a switchable DC voltagesource 945, by means of which, as necessary, a DC voltage ofpredeterminable magnitude can be applied between the electrode layers,such that the piezoactive layer 940 is permeated by an electric fieldand its layer thickness z changes in a manner dependent on the appliedvoltage.

The active layer 940 and the adjoining electrode layers 943, 942 are ineach case applied by means of pulsed laser deposition (PLD). Withcorresponding method implementation, the layers can be applied asmonocrystalline layers, such that the surface of the upper electrodelayer 942 to which the Mo/Si layer pairs are subsequently applied hassuch a low roughness that subsequent polishing can be obviated.

Some variants for using structured electrodes in the construction ofmirror arrangements are elucidated in connection with FIGS. 10A to 10C.FIG. 10A shows in oblique perspective a schematic view of threecross-sectionally rectangular mirror elements 1010, 1011, 1012 of amirror arrangement 1000, which can be used, for example, as a fieldfacet mirror in the region of a field plane of the illumination systemof an EUV projection exposure apparatus (cf. FIG. 11). The layerconstruction of the mirror element 1010 is illustrated in detail. In amanner similar to that in the case of the arrangement from FIG. 1, amultilayer arrangement 1030 composed of many individual layers isapplied to a substrate 1020. An individual active layer 1040 composed ofpiezoelectric active crystalline layer material is arranged between afirst layer group 1031 situated in the vicinity of the radiationentrance surface and a second layer group 1032 near the substrate. Bothlayer groups in each case consist of a plurality of (e.g. between 10 and30) layer pairs with a suitable layer period and have in each case bythemselves a reflective effect on the penetrating EUV radiation. Bymeans of the electrically alterable layer thickness z of the activelayer 1040, it is possible to set a specific phase shift that can be setcontinuously variably with regard to its extent between the secondportion reflected by the second layer group 1032 and the first portionreflected by the first layer group 1031.

The electrode arrangement for driving the active layer has asubstrate-side second electrode layer 1043 which is continuous over theentire cross section of the mirror element and which is connected to onepole of a settable DC voltage source 1045. The first electrode layer1042 arranged on the opposite surface of the active layer 1040 isdesigned as a structured layer electrode and subdivided into a pluralityof electrode segments 1042A, 1042B which are situated alongside oneanother and are electrically insulated from one another. Each of theelectrode segment covers only a fraction of the total cross-sectionalarea of the mirror element, e.g. less than 50% or less than 40% or lessthan 30% or less than 20% or less than 10%. In general, the area of anindividual electrode segment is at least 1% or at least 5% of the totalarea of the structured layer electrode. A narrow insulating section 1044composed of an electrically non-conductive layer material isrespectively situated between adjacent electrode segments. Theinsulating regions in each case run obliquely with respect to the longedge (x-direction) and obliquely with respect to the short edge(y-direction) of the mirror element. Further orientations are alsopossible.

The structured electrode 1042 can be produced with the aid ofmicrolithographic methods, for example.

Each of the electrode segments is connected to the other pole of the DCvoltage source 1045 via a separate electrical line and can be put at asuitable potential relative to the continuous second electrode layer1043 independently of the other electrode segments. In general, there isa separate switchable or continuously variably settable DC voltagesource for each electrode segment.

With the aid of the structured electrode it is possible to alter thelayer thickness of the active layer 1040 in a location-dependent mannerin order to produce a layer thickness profile extending in thex-direction. In accordance with the layer thickness varying in thex-direction, a phase shift of the reflected radiation portions whichvaries locally in the x-direction is then established, such that, as aconsequence thereof, the reflectivity R of this individual mirrorelement can also be set and varied in a location-dependent manner in thex-direction. The schematic diagram shown above the mirror arrangementshows that a higher total reflectivity R is set in the region of theelectrode segments shown on the left than in the region of the oppositenarrow side, wherein a transition region is situated between them.

In the case of this mirror arrangement 1000 it is thus possible not onlyin each case to individually control the reflectivity level ofindividual mirror elements 1010, 1011, 1012, but also to set a desiredprofile with varying local reflectivity R within each individual mirrorelement. Consequently, an individual mirror element in turn forms an EUVmirror arrangement having two or more mirror elements that can be setindividually with respect to their reflectivity, wherein the form andsize of said mirror elements are determined by the form and size of theelectrode segments 1042A, 1042B.

FIGS. 10B and 10C schematically show other structuring geometries ofstructured layer electrodes which can be used in embodiments ofindividual mirror elements or of mirror elements of a mirror arrangementequipped with a plurality of mirror elements. The overall circularstructured electrode 1050 can be used in a cross-sectionally circularmirror element in combination with a likewise circular counterelectrodewhich, however, is not subdivided into segments. In the case of theexample, the structured electrode has twelve individually driveableelectrode segments 1050A, 1050B of identical form and size, which eachcover an angular range of approximately 30°. A contact point KP forconnecting an electrical connection to a voltage source is provided ineach case at the free outer edge of an electrode segment. Such astructured electrode arrangement can be provided, for example, on anindividual mirror of a projection lens in order to set a radiallysymmetrical non-uniform distribution of the reflectivity and/or of thereflected phase at the mirror surface, wherein the reflection behaviourcan in each case vary in the azimuthal direction (circumferentialdirection) and this variation can be set in a targeted manner. By way ofexample, it is possible to set a local reflectivity distribution havinga two-fold or threefold or four-fold or six-fold azimuthal symmetry.

The structured electrode 1060 in FIG. 10C has a multiplicity ofelectrode segments 1060A, 1060B which are electrically insulated fromone another and which, with a small mutual distance between them, coverthe circular area of the mirror element. Instead of the chequeredarrangement of square electrode segments, by way of example it is alsopossible to provide other polygonal shapes, for example triangles orhexagons. The counterelectrode (not shown) on the other side of theactive layer is continuous, that is to say not subdivided into segments.Insulating regions 1064 composed of electrically insulating material andextending in different directions are arranged between the respectivesquare electrode segments. Those electrode segments which adjoin thecircular outer edge of the electrode arrangement can be directlycontact-connected laterally from outside via corresponding first contactpoints KP1. The inner electrode segments without connection to the outerside of the structured electrode layer are contact-connected via narrowlines which run in an electrically insulated manner on both sides withinthe insulating regions 1064 to the electrode segments with which contactis respectively to be made and to second contact points KP2. It isthereby possible to put each electrode segment at a specific potentialrelative to the counterelectrode (not shown) separately andindependently of other electrode segments and thereby to set the layerthickness of the active layer into the associated layer range.

EUV mirror arrangements which, with the aid of piezoelectrically activelayers, enable a spatially resolving setting of the reflectivity profileover the total mirror surface of the mirror arrangement and/or over thesurface of an individual mirror element can be used for various tasks.Possible uses in the context of the illumination system for an EUVmicrolithography projection exposure apparatus are presentedhereinafter.

FIG. 11 shows optical components of an EUV microlithography projectionexposure apparatus 1100 for exposing a radiation-sensitive substratearranged in the region of an image surface 1160 of a projection lens1130 with at least one image of a pattern of a reflective patterningdevice or mask arranged in the region of an object surface 1120 of theprojection lens.

The apparatus is operated with radiation from a primary radiation source1114. An illumination system 1110 serves for receiving the radiationfrom the primary radiation source and for shaping illumination radiationdirected onto the pattern. The projection lens 1130 serves for imagingthe structure of the pattern onto a light-sensitive substrate.

The primary radiation source 1114 can be, inter alia, a laser plasmasource or a gas discharge source or a synchrotron-based radiationsource. Such radiation sources generate a radiation 520 in the EUVrange, in particular having wavelengths of between 5 nm and 30 nm. Inorder that the illumination system and the projection lens can operatein this wavelength range, they are constructed with components that arereflective to EUV radiation.

The radiation 1120 emerging from the radiation source 1114 is collectedby means of a collector 1115 and directed into the illumination system1110. In this case, the radiation passes through an intermediate focalplane 1122, in which devices for separating undesired radiation portionscan be provided. The illumination system comprises a mixing unit 1112, atelescope optical unit 1116 and a field shaping mirror 1118. Theillumination system shapes the radiation and thus illuminates anillumination field situated in the object plane 1150 of the projectionlens 1130 or in the vicinity thereof. In this case, the form and size ofthe illumination field determine the form and size of the effectivelyused object field in the object plane 1150.

A reflective reticle or some other reflective patterning device isarranged in the object plane 1150 during operation of the apparatus. Theprojection lens in this case has six mirrors and images the pattern ofthe patterning device into the image plane, in which a substrate to beexposed, e.g. a semiconductor wafer, is arranged.

The mixing unit 1112 substantially consists of two facet mirrors 1170,1180. The first facet mirror 1170 is arranged in a plane 1172 of theillumination system that is optically conjugate with respect to theobject plane 1150. It is therefore also designated as a field facetmirror. The second facet mirror 1180 is arranged in a pupil plane 1182of the illumination system that is optically conjugate with respect to apupil plane of the projection lens. It is therefore also designated as apupil facet mirror.

With the aid of the pupil facet mirror 1180 and the imaging opticalassembly which is disposed downstream in the beam path and whichcomprises the telescope optical unit 1116 and the grazing incidencefield shaping mirror 1118, the individual mirroring facets (individualmirrors) of the first facet mirror 1170 are imaged into the object field1152.

By means of the facets of the field facet mirror 1170, on the one hand,and of the pupil facet mirror 1180, on the other hand, the radiationbeam coming from the radiation source is split into a plurality ofillumination channels, wherein each illumination channel is assignedexactly one pair of facets comprising a field facet and a pupil facet.The following components guide the radiation of all the illuminationchannels to the object field 1152.

The spatial (local) illumination intensity distribution at the fieldfacet mirror determines the local illumination intensity distribution inthe object field. The spatial (local) illumination intensitydistribution at the pupil facet mirror 1180 determines the illuminationangle intensity distribution in the object field.

EUV projection exposure apparatuses having a similar basic constructionare known e.g. from WO 2009/100856 A1 or WO 2010/049020 A1, thedisclosure of which is incorporated by reference in the content of thisdescription.

In the embodiment shown, the channel-dependent transmissions and hencethe energetic illumination angle distribution can be influenced by meansof the reflectivities of the individual field and pupil facets. Thespatial illumination intensity distribution in the object field can beinfluenced by location-dependent variation of the field facetreflectivities.

Each of the facet mirrors 1170, 1180 is an EUV mirror arrangement havinga multiplicity of individual mirror elements. The mirroring frontsurfaces thereof are designated as element mirror surfaces and form thefacets (mirror surfaces) of the facet mirror.

The field facet mirror 1170 and the pupil facet mirror 1180 areconstructed in the manner of the EUV mirror arrangement 100 shown inFIG. 1. Each of the mirror elements therefore has a multilayerarrangement, into which an individual piezoelectrically active layersituated between two electrode layers is arranged between a second layergroup near the substrate and a first layer group near the surface. Theelectrode layers are electrically connected to a control device 1190,which is configured for applying electrical voltage selectively asnecessary to the electrode pairs associated with the individual activelayers, in order to vary the layer thickness. This can be done for eachmirror element separately and independently of the other mirrorelements, such that different local distributions of the layer thicknessvariation and thus different local reflectivity distributions can be setat the relevant facet mirror.

The possibility of precisely controlling the local reflectivity of thefacet mirrors can be used for controlling the illumination intensitydistribution by way of the pupil of the illumination system and in theillumination field. If, in one or more of the illumination channels, thereflectivity of an associated field facet and/or the reflectivity of anassociated pupil facet are/is altered by electrical driving, then it ispossible to alter the illumination intensity in said illuminationchannel in a targeted manner within a certain setting range. Since thisis possible for a plurality and, if appropriate, all of the illuminationchannels independently of other illumination channels, it is possible torealize a controllable manipulator for the illumination intensitydistribution by way of the pupil in order to provide in the illuminationfield exactly a desired intensity distribution in a manner dependent onthe illumination angle.

The field facet mirror can also have mirror elements which areconstructed with structured electrodes and thus enable alocation-dependent setting of the reflectivity of each individual mirrorelement (cf. FIG. 10). Since, in this embodiment, it is possible tocontrol the reflectivity of the field facet mirror 1170 within theindividual facets in a location-independent manner, it is possible toapproximate the local illumination intensity distribution in theillumination field 1152 that is optically conjugate with respect theretoto the desired illumination intensity distribution in alocation-dependent manner. In this way, a desired value of the fielduniformity can be set precisely.

In the image field, the images of the individual field facets aresuperimposed. The long side thereof runs parallel to the x-direction(cross-scan direction), while the short side runs parallel to they-direction, which corresponds to the scanning direction in scannersystems. What is achieved by the inclination of the insulating sections1044 (FIG. 10A) is that the projection thereof into the image plane runsneither parallel nor perpendicularly to the scanning direction, butrather obliquely with respect thereto. During the scanning process, aneffect integrated over the scanning process in the y-direction arises inthe image field, such that in the image field possible artefacts of theinsulating sections practically do not appear.

A fully programmable “neutral filter” for intensity distributions in thepupil and field of the illumination system is thus realized. In thiscase, the lateral resolution of the controllable neutral filter isdetermined by the lateral extent of the mirror elements which aredriveable separately from one another, or by the number thereof over theilluminated cross section.

In the embodiment, the illumination intensity distribution in theillumination field 1152 is monitored by a field uniformity sensor 1153and the illumination intensity distribution in the pupil surface ismonitored by means of a pupil intensity sensor 1183. In a departure fromthe schematic illustration, these sensors can be situated in the regionof the image plane of the projection lens. They are connected to thecontrol device 1190, which, on the basis of the sensor signals, controlsthe voltage between the electrode layers of the individualpiezoelectrically active layers and thus the local reflectivitydistribution of the facet mirrors. A great precision of the importantillumination parameters in the pupil and field of the illuminationsystem is permanently ensured by this control loop.

Further embodiments now explained in connection with FIGS. 12 and 13utilize some properties provided by the specific layer structure of EUVmultilayer mirrors including at least one active layer made of apiezoelectrically active layer material. Firstly, single layers or layerstacks in EUV multilayer arrangement typically exhibit significantelectrical conductivity, the specific electric resistance often being inthe range of about 3×10⁻⁶ Ω×cm. Therefore, layers of the multilayerarrangement may be utilized as contact layers, as explained in someembodiments above. Secondly, activation of the piezoelectric activematerial is effected by an electric field so that it is not necessary toestablish electrically conducting contact between electrodes and thePiezo electrically active material.

The EUV mirror arrangement 1200 according to the embodiment shown inFIG. 12 comprises a substrate 1220, a second layer group 1243 formed onthe substrate, an active layer 1240 made of a Piezo electrically activelayer material formed on the second layer group 1243, and a first layergroup 1242 formed on the active layer 1230 on the radiation incidentside of the mirror arrangement. Each layer group may consist of multiplelayer pairs with alternating high and low refractive index material,such as Mo/Si pairs or Ru/Si pairs. The second layer group 1243 islaterally subdivided into smaller portions 1243A, 1243B, 1243C, eachsmaller portion defining the lateral dimensions of a mirror element1210, 1211, 1212 etc. Adjacent smaller portions of the layer groups areelectrically separated from each other by interposed insulating sections1244 formed between directly adjacent smaller portions of the secondlayer group. Each smaller portion forms a segment of a structuredelectrode.

In contrast to this, the layers of the first layer group 1242 extendcontinually (without interruption) over the entire useful cross sectionof the mirror arrangement. This embodiment utilizes the fact that thelayer materials forming the layer pairs in the multilayer arrangementhave sufficient electrical conductance so that the layer groups may beused as electrodes to provide a required electrical field across theactive layer 1240. To this end, some or all individual layers of thefirst layer group 1242 on the radiation incidence side are electricallyconnected to one output of an electric voltage source 1245. Each of thesmaller portions 1243A to 1243C is electrically connected through thesubstrate 1220 to a separate output of the voltage source 1245. In thiscircuitry the first layer group 1242 functions as a common electrode foreach single mirror of the multiplicity of mirror elements in the mirrorarrangement. Individual voltage values U₁, U₂ and/or U₃ may be setbetween the reference potential of the common electrode 1242 and theindividual electrode segments 1243A, 1243B, 1243C on the opposite sideof the active layer 1240.

Mirror arrangements such as shown in the embodiment 1200 may bemanufactured in a manufacturing process as follows. In a first step, thelayers of the second layer group 1243 may be formed on the substrate ascontinuous layers. In a second step, the layer arrangement may bestructured by a suitable process, such as a lithographic process, tosegment the layers laterally and to provide a space for the insulatingmaterial forming the insulating sections 1244 in any desired geometry sothat smaller portions having the required shaped (for example quadratic,hexagonal, triangular etc.) may be formed. The insulating material maythen be inserted in a suitable process. After that, all subsequentlayers on the radiation incidence side may be formed by a suitableprocess as continuous layers extending over the entire useful area ofthe mirror arrangement. Any residual structural roughness present on thefree surface after forming the structured second layer group 1243 may beevened out by the subsequent coating layers so that a large useful areawith homogeneous properties may be obtained. It may be noted that use ofa continuous layer group extending across the entire useful area (firstlayer group 1242) as a common electrode layer helps to reduce the numberof electrical contacts required to connect the individual mirrorelements electrically.

The layer structure of the mirror arrangement may include additionallayers to those mentioned in connection with FIG. 12. For example, oneor more insulating layer made of electrically insulating material may beformed between the electrode layer groups 1242, 1243 and thepiezoelectrically active layer 1240 if required.

In a variant of the embodiment shown in FIG. 8 above, each of the pairsof electrode layers sandwiching an interposed active layer is connectedto the same DC voltage source 845, thereby allowing adjusting theindividual thicknesses of the active layers synchronously by the samerelative amounts. A similar result may be achieved by using a commonreference electrode providing a common reference potential, similar tothe common first electrode formed by the first layer group 1242 in FIG.12. In such arrangement, the active layers are electrically connected inseries. The arrangement works properly if the field intensity of theelectrical field is sufficiently strong across all Piezo electriclayers.

The embodiment of an EUV mirror arrangement 1300 in FIG. 13 may beconsidered as a variant of the embodiment shown in FIG. 8. In the layerstructure formed on substrate 1320, active layers 1340 made ofpiezoelectric material are alternatingly arranged with interposedrelatively thin layers 1336 made of a material having a real part of therefractive index which is smaller than the real part of the refractiveindex of the piezoelectric material forming the thicker active layers1340. The thinner layers 1336 are electrically conductive and serve aselectrode layers influencing the respective active layers 1340interposed between the pairs of electrode layers.

While the same voltage is applied between adjacent electron layers inthe embodiment of FIG. 8, the variant of electrical connection shown inFIG. 13 allows adjusting the thickness of each of the active layers 1340individually. To this end, the electrode layers 1336 enclosing eachindividual active layer 1340 are electrically connected to individualoutputs of a voltage source so that the electrical field strengthaffecting each of the active layers 1340 may be set independent of thefield strength acting on other active layers. As a result, the thicknessof each of the active layers 1340 may be set individually and may bedifferent from the thickness of other active layers in the layer stack.Depending on the individual voltages U₁, U₂, . . . , U_(n-1) set betweenadjacent electrode layers, the layer structure may deviate from astrictly periodic sequence of layers so that the individual mirrorelements may exhibit relatively high reflectance for a wider range ofangles of incidence. In other words, a spectral broadband response ofthe EUV mirror arrangement may be obtained and a suitable compromisebetween maximum reflectivity and sufficient broadband response may beadjusted by setting the individual voltages of the electrode pairsseparately.

In a variant of the embodiment shown in FIG. 9 the individual electrodes942 formed on the light incidence side of the active layers 940 may bereplaced by a single continuous top electrode formed by the third layergroup 933 in a manner similar to the construction shown in FIG. 12. Inother words, the individual electrodes 943 on the substrate side may beformed as shown and separated from the laterally adjacent electrodes byinsulating sections similar to those shown in FIG. 12. A similar layercan also be obtained by replacing the individual multi layer electrodes1243A to 1243C in FIG. 12 by a single electrode layer made ofelectrically conductive material.

A further embodiment not shown in detail here may be derived from theembodiment of FIG. 10A, which comprises a structured electrode 1042 onone side of the active layer 1040. As discussed above, the structuredelectrode 1042 is subdivided into a suitable number of individualelectrode segments 1042A, 1042B which are laterally separated byinsulating sections 1040 and which are electrically insulated from eachother. The electrical insulation may be achieved by forming a continuousinsulating layer directly on the structured electrode.

In a variant of the embodiment shown in FIG. 10A, the structuredelectrode is formed on the substrate side of the active layer, andconnected to individual outputs of a voltage source similar to themanner shown in FIG. 12. A common reference electrode common to allindividual mirror elements may be provided on the opposite side of theactive layer in a manner similar to that shown by the common electrode1242 in FIG. 12.

Some possible uses of EUV mirror arrangements in the illumination systemof a microlithography projection exposure apparatus have been explainedon the basis of the exemplary embodiments. Alternatively oradditionally, provision can also be made for determining at least onemirror of the telescope optical unit 1116 of the illumination systemand/or at least one mirror of the projection lens 1130 in accordancewith an embodiment of an EUV mirror arrangement.

The total mirror surface of an EUV mirror arrangement can be flat in themanner of a plane mirror. It is also possible to design an EUV mirrorarrangement in a convexly or concavely curved mirror surface. In theexamples, the individual mirror surfaces are plane surfaces in eachcase. This is not mandatory, however. Individual or all of theindividual mirror surfaces of the mirror elements can also be convexlyor concavely curved.

In the exemplary embodiments, the relative orientations of theindividual mirror surfaces of the EUV mirror arrangement are defined ineach case, wherein the electrically induced change in layer thickness ofthe active layer merely leads to a raising or lowering of said mirrorsurface with respect to the substrate. It is additionally also possiblefor individual or all of the mirror elements of an EUV mirrorarrangement to be tiltable relative to one another with the aid ofindependent actuators, in order to alter the illumination angledistribution of the reflected radiation in a targeted manner (cf. e.g.WO 2009/100856 A1).

The shape of the individual mirror elements can be adapted to thedesired application. If, by way of example, an EUV mirror arrangement isintended to be used as a field facet mirror, the individual mirrorsurfaces can be rectangular with a predeterminable aspect ratio or elsearcuately curved. In the case of EUV mirror arrangements which areintended to be used as a pupil facet mirror, round cross sections of theindividual mirror elements can also be useful besides polygonal crosssections.

The layer construction of the multilayer arrangement in the region ofthe successive layer pairs can be adapted to the application strivenfor. If high maximum reflectivities are required for a relatively smallangle-of-incidence range, then a fully periodic sequence of layer pairscan be advantageous. If, by contrast, a broadband configuration in theangle space and/or a spectral broadband configuration are/is desired,layer pairs with different periods can also be combined (cf. e.g. DE 10155 711 B4 or WO 2010/118928 A1). In order to reduce the dependence ofthe reflectivity on the angle of incidence, the layer arrangement can,in principle, also be constructed in the manner disclosed in U.S. Pat.No. 7,382,527 B2. In particular, different material pairings can beprovided for the layer pairs of a multilayer arrangement.

The described layer constructions of the multilayer arrangements can, inprinciple, also be provided in the case of mirrors having only a singlemirror element. It is thereby possible, e.g. by means of changing thelayer period of a layer arrangement in an electrically induced manner,to effect an adaptation to a slightly different central wavelengthand/or an adaptation to changed angles of incidence. Moreover, theglobal intensity or dose can be adapted.

When using structured layer electrodes (see e.g. FIG. 10), alocation-dependent control of the reflectivity and/or phase of theradiation impinging on an individual mirror element is possible.

Embodiments of the invention can be used not just in optical systems forprojection microlithography. By way of example, use in the field ofX-ray microscopy is possible, particularly in the field of EUV maskmetrology. By way of example, one or more mirror arrangements can beused in an Aerial Image Monitoring System (AIMS) or in an ActinicPatterned Mask Inspection System (APMI) or in an Actinic BlankInspection System (ABI). Lenses for EUV AIMS systems are disclosed e.g.in the international publications WO 2011/012267 A1 and WO 2011/012266A1. Applications in EUV system metrology, e.g. in an actinic systeminterferometer, are likewise conceivable. Applications in the field ofEUV astronomy and for optical assemblies in synchrotron systems or FELbeam lines (FEL=Free Electron Laser) are furthermore conceivable.

The exemplary embodiments were configured for a central wavelength of13.5 nm. Other exemplary embodiments can be optimized for otherwavelengths (wavelength ranges), for example for a central wavelength ofapproximately 6.8 nm. In this case, in particular, it is also possibleto use other layer materials for the alternate layers of the layerpairs, e.g. the combination La/B₄C. In the case of shorter centralwavelengths it may be expedient to increase the numbers of layer pairsin layer groups in comparison with the examples described above.

The above description of various embodiments has been given by way ofexample. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. The applicant seeks, therefore, tocover all such changes and modifications as fall within the spirit andscope of the invention, as defined by the appended claims, andequivalents thereof.

The invention claimed is:
 1. A mirror for extreme ultraviolet (EUV)radiation comprising a mirror element which forms a mirror surface ofthe mirror, and which comprises: a substrate, a multilayer arrangementapplied on the substrate and having a reflective effect with respect tothe EUV radiation, said multilayer arrangement comprising: amultiplicity of layer pairs having alternate layers composed of a highrefractive index layer material and a low refractive index layermaterial, an active layer arranged between a radiation entrance surfaceand the substrate and consisting of a piezoelectrically active layermaterial, the layer thickness of which active layer alters by action ofan electric field; and an electrode arrangement configured to generatethe electric field acting on the active layer, wherein the electrodearrangement, for driving the active layer has an electrode layer,wherein the electrode layer is a structured layer electrode and issubdivided into a plurality of electrode segments which lie alongsideone another and are electrically insulated from one another, and whereineach of the electrode segments covers only a fraction of a totalcross-sectional area of the mirror element.
 2. The EUV mirror accordingto claim 1, wherein each of the electrode segments covers less than 50%and more than 1% of the total area of the structured layer electrode. 3.The EUV mirror according to claim 1, wherein the electrode arrangementcomprises a common electrode opposite to the structured layer electrode,the common electrode extending over at least a plurality of electrodesegments.
 4. The EUV mirror according to claim 3, wherein the commonelectrode is formed on a radiation incidence side of the active layer,opposite to the substrate side of the active layer.
 5. The EUV mirroraccording to claim 4, wherein the common electrode arranged on theradiation incidence side comprises a plurality of single layers stackedon top of each other to form a multilayer.
 6. The EUV mirror accordingto claim 1, wherein the mirror element is circular in cross section andthe electrode layer is an overall circular structured layer electrodesubdivided into the electrode segments each covering a predefinedangular range, whereby a reflection behavior of the mirror elementvaries circumferentially in a predetermined manner.
 7. The EUV mirroraccording to claim 1, wherein the structured electrode has a pluralityof polygonal electrode segments forming a checkered pattern.
 8. The EUVmirror according to claim 1, wherein the multilayer arrangementcomprises a first layer group arranged between the radiation entrancesurface and the active layer and having a first number N1 of layerpairs, and a second layer group arranged between the active layer andthe substrate and having a second number N2 of layer pairs, wherein thenumbers N1 and N2 of layer pairs of the first layer group and of thesecond layer group are configured such that, for at least one angle ofincidence of the radiation impinging on the radiation entrance surface,the first layer group transmits a portion of the incident radiationthrough the active layer to the second layer group and the radiationreflected by the multilayer arrangement contains a first portionreflected by the first layer group and a second portion reflected by thesecond layer group.
 9. The EUV mirror according to claim 8, wherein theactive layer, absent the electric field, has a layer thicknessconfigured such that for a reference angle of incidence of the incidentradiation a reflectivity of the multilayer arrangement is altered by amaximum of 20% when the active layer is subjected to the electric field.10. The EUV mirror according to claim 8, wherein the piezoelectricallyactive layer material substantially consists of barium titanate(BaTiO₃).
 11. The EUV mirror according to claim 8, wherein at least oneof the following conditions holds true:10<N1<30  (1)15<N2<50  (2)30<(N1+N2)<70 and N1>10 and N2>10  (3)N1≤N2  (4)z≥2 nm  (5)z≤35 nm  (6)Δz≥0.1 nm  (7)0.15 nm≤Δz≤2 nm,  (8) where z is the layer thickness of the active layerand Δz is a change in layer thickness produced by the generated electricfield.
 12. The EUV mirror according to claim 1, wherein the multilayerarrangement has a multiplicity of active layers composed of thepiezoelectrically active layer material, wherein the active layers arerespectively arranged alternatively with non-active layers.
 13. The EUVmirror according to claim 12, wherein the active layer material consistspredominantly of a ceramic material of the type (Li, Na, K)(Nb, Ti)O₃.14. The EUV mirror according to claim 12, wherein individual non-activelayers formed by electrically conductive, non-piezoelectrically activelayer material are connected to individual outputs of a voltage source,such that electric field strengths affecting the different active layersare set individually.
 15. The EUV mirror according to claim 1, whereinthe multilayer arrangement has a third layer group arranged between theradiation entrance surface and the active layer and having a thirdnumber N3 of layer pairs, wherein the third number N3 is selected suchthat, for at least one angle of incidence of the radiation impinging onthe radiation entrance surface, the third layer group reflects orabsorbs the incident radiation before the incident radiation reaches theactive layer.
 16. The EUV mirror according to claim 1, wherein theelectrode arrangement further comprises a second electrode layer, andthe active layer is arranged between the electrode layers.
 17. The EUVmirror according to claim 1, wherein the active layer is a PLD layerapplied by pulsed laser deposition (PLD).
 18. The EUV mirror accordingto claim 1, wherein at least one of: at least one electrode layer is aPLD layer applied by pulsed laser deposition (PLD), and at least oneelectrode layer consists of an electrically conductive ceramic material.19. The EUV mirror according to claim 1, wherein the piezoelectricallyactive layer material is selected from the group consisting of:Ba(Sr,Zr)TiO₃, Bi(Al,Fe)O₃, (Bi,Ga)O₃, (Bi,Sc)O₃, CdS, (Li,Na,K)(Nb,Ta)O₃, Pb(Cd,Co,Fe,In,Mg,Ni,Sc,Yb,Zn,Zr) (Nb,W,Ta,Ti)O₃, ZnO, ZnSor contains at least one material of this group in combination with atleast one other material.
 20. An optical system comprising at least oneEUV mirror as claimed in claim
 1. 21. The optical system according toclaim 20, wherein the optical system is an illumination system or aprojection lens of a micrography projection exposure apparatus.
 22. Amethod for operating an optical system comprising at least one EUVmirror as claimed in claim 1, comprising varying a local reflectivitydistribution over the mirror surface of the mirror element in alocation-dependent manner by selectively driving at least individualones of the active layers of the mirror elements via the correspondingones of the electrode segments of the structured layer electrode.